1. Field of the Invention
The invention relates to novel 3-substituted oxetane monomers for photoinitiated cationic polymerization, to compositions for polymerization, containing these oxetanes, to processes for polymerizing the monomers, and to polymers produced thereby.
2. Information Disclosure
Ultraviolet (UV)-induced polymerization, or curing, has become very important in the wood coating, metal decorating and printing industries as a consequence of its high cure and application speeds, essentially pollution-free operation, very low energy requirements and generally excellent properties. Early developments in this field centered about the photoinduced free radical polymerization of multifunctional acrylates and unsaturated polyesters. Today, these materials still remain the workhorses of the UV cure industry. While the bulk of the current research effort continues to be directed toward photoinduced free radical polymerizations, it is well recognized that ionic photopolymerizations also hold considerable promise in many application areas. Photoinduced cationic polymerizations are particularly attractive because of the wealth of different chemical and physical properties which can potentially be realized through the polymerization of a wide variety of monomers. Further, photoinitiated cationic polymerizations have the advantage that they are not inhibited by oxygen and thus, may be carried out in air without the need for blanketing with an inert atmosphere to achieve rapid and complete polymerization.
Until the present, photoinitiated cationic polymerization technology has centered about the photopolymerization of two types of monomers: epoxides and vinyl ethers. In particular, the photopolymerization of epoxides gives coatings with high thermal capability, excellent adhesion and good chemical resistance. Conventional photocurable epoxides have, however, the drawback that they undergo photopolymerization at rather slow rates. This factor renders them unsuited for certain applications such as paper and plastic coatings in which rapid UV curing is required.
Eckberg et al. [Radtech '90 North American Proceedings vol 1, 358-370 (1990)], which is incorporated herein by reference, discloses a series of linear and branched silicones having strained epoxide functional groups for photopolymerization. Catalysts and polymerization conditions are described.
Crivello and Lee [Radtech '90 North American Conference Proceedings Vol. 1, 432-445 (1990)], which is incorporated herein by reference, have described silicon-containing multifunctional monomers bearing strained cycloaliphatic epoxide rings that undergo especially rapid polymerization. These cationic photopolymerizations occur at speeds at least 100 times faster than the fastest commercial epoxide monomers. Similar silicon-containing monomers bearing open chain epoxides display rather poor UV curing responses.
Based on those observations, Crivello and Lee concluded that a major factor in the high photoresponse of the silicon-containing epoxides bearing cycloaliphatic epoxide rings is the high ring strain of the epoxide groups in these compounds. It is well known that epoxides have the highest ring strain of the cyclic ethers. Thus, all types of epoxides would be presumed to be more reactive in cationic UV photopolymerization than oxetanes, which would be more reactive than monomers containing five membered rings i.e. ##STR2## Further, it would also be expected that multifunctional oxetane monomers would be less reactive than the corresponding multifunctional epoxides. Oxetanes are known to undergo polymerization but their rates of polymerization are not discussed in the art in which the oxetanes are disclosed.
U.S. Pat. No. 4,058,400 (Crivello) discloses that 3,3-bischloromethyloxetane and alkoxyoxetanes may be cationically photopolymerized.
U.S. Pat. No. 3,673,216 (Schroeter) discloses a series of 4,4-dialkyl-2-alkoxyoxetanes and states that they may be polymerized with Friedel-Crafts catalysts.
German Patent 1,021,858 (Bodenbenner and Wegler) discloses oxetanes of general formula ##STR3## wherein R is an aromatic residue having two or more valences and R.sup.1 is ethyl. Specifically disclosed are compounds wherein R is ##STR4##